Phthalocyanines and their sulfonated derivatives show considerable promise to replace hematoporphyrin derivatives for photodynamic therapy (PDT) of human tumors. The ability of tissue-omnipresent ascorbic acid (Vitamin C) to deactivate photoexcited phthalocyanine with consequent formation of the electron spin resonance detectable ascorbate radical, brings new perspective to the understanding of the chemical mechanisms of photodynamic reactions. Our studies with chloraluminum phthalocyanine tetrasulfonate and riboflavin (Vitamin B2) as photosensitizers demonstrate free radical formation by direct reaction between ascorbate ion and the excited triplet state of the photosensitizer. This process is in competition with the reaction of ascorbate ion with singlet oxygen, a reaction which does not lead to the formation of ascorbate radicals. The chemical effects of ultrasound were investigated in relation to sonodynamic cancer therapy, a new approach in which the synergistic effects of combined ultrasound and anticancer drugs (e.g. Adriamycin) have been observed. The 50 kHz sonolysis of argon-saturated dimethyl sulfoxide-water mixtures was studied by electron spin resonance and spin trapping with 3,5-dibromo-4-nitrosobenzenesulfonate. While for previous sonolysis studies of aqueous mixtures of solvents more volatile than water, spin-adducts yields decrease at high mole fractions of the organic solvent because of the decreasing effective ratio of the specific heat at constant pressure to the specific heat at constant volume in the imploding argon cavitation bubbles, the pyrolysis product of dimethylsulfoxide increases with increasing dimethylsulfoxide concentrations.